A wind turbine comprises two, three or more rotor blades arranged at a hub. A rotor blade is as a rule adjustable relatively to the hub wherein the so called pitch of the rotor blade is adjusted. With the adjustment of the pitch of the rotor blades e.g. the operation of the wind turbine can be influenced and controlled.
In U.S. Pat. No. 4,352,629, DE 27 15 584 A1, U.S. Pat. Nos. 4,363,241, 4,431,375 and 4,545,728 wind turbines are disclosed each comprising a pitch mechanisms wherein subject to the impact of the wind to the rotor blades and thus to the respective pitch mechanism the pitch of a rotor blade of the wind turbine is altered in particular to avoid an overspeed of the rotor.
For the adjustment of the pitch of a rotor blade in some wind turbines a hydraulic pitch cylinder is used.
In U.S. Pat. No. 6,327,957 B1 a wind turbine is disclosed comprising a wind turbine rotor with oppositely disposed flexible blades attached to the hub of a drive shaft by a flexible spar. The blades are pitched to a stall position by a collective pitch mechanism in the absence of proper operating conditions. The pitch mechanism is used to change the pitch of the blades collectively. The pitch mechanism comprises a pitch beam extending transversely of the axis of the drive shaft and having a shaft portion that reciprocates within the hollow portion of the drive shaft. Opposite ends of the pitch beam are coupled to respective blade root-ribs by linkages. Movement of the pitch beam inwardly of the drive shaft is provided by a hydraulic cylinder pitch actuator on a pitch yoke attached to the hub. Opposite movement of the pitch beam, outwardly of the drive shaft, is provided by a strong compression spring at the innermost end of the shaft portion of the-pitch beam. To pitch the blades to the run position, the hydraulic cylinder of the pitch actuator is charged from a hydraulic fluid supply to move the pitch beam inwardly of the drive shaft, so that wind in the downwind direction drives the rotor. To pitch the blades to the stall position, the hydraulic pressure in the cylinder is released, and the strong compression spring drives the pitch beam outwardly of the drive shaft. A thin mechanical tube guides the shaft portion of the pitch beam inside the drive shaft and positions the powerful coil spring against the end of the shaft portion of the pitch beam to permit pre-loading of the blade pitch system. In a shut-down mode, the blades are held in the stalled position by the spring force.
In another wind turbine the hydraulic pitch cylinder comprises a piston rod movable in a tube of the hydraulic pitch cylinder and acts on an adjusting plane e.g. in form of a rotatable plate connected with the rotor blade. E.g. the piston rod is connected to the rotatable plate and the hydraulic pitch cylinder tube is fastened at a support which is fastened at the hub. Thus when the piston rod is hydraulically moved along the hydraulic pitch cylinder tube the pitch of the corresponding rotor blade is adjusted.